It is well known that the general trend in semiconductor integrated circuit fabrication is to produce smaller and smaller devices so that more circuitry can be packed into less wafer real estate. This trend means that the spacing between devices of an integrated circuit shrinks. As the spacing between devices shrinks, adjacent devices increasingly interact with each other, reducing performance of the integrated circuit.
The basic measurement parameter that characterizes this interaction effect is called electrical isolation. If high electrical isolation, and in particular, high RF isolation, between devices of an integrated circuit can be achieved, those devices can be spaced more closely together. As a result, the die size of the integrated circuit can be minimized. Minimum size, in turn, implies smaller packaging and, thus, less space used on the application PCB Board. Also, since wafer processing costs are nearly independent of the chip size, smaller circuits imply more die per wafer and thus a lower cost per die.
Isolation is particularly important for analog integrated circuits and for analog/digital mixed integrated circuits such as those used in wireless and wireline communications applications. In general, wireless communication devices use high-frequency signals such as 900 MHz to 1900 MHz for cellular phones, and higher frequencies (up to 6 GHz or more) for other applications such as wireless LANs and fiber optic transceivers.
RF signals at such frequencies are difficult to generate and control. These signals also have a tendency to interfere with one other because they are easily coupled by parasitic properties present in all electronic components, including integrated circuits. Such undesirable parasitic effects result, for example, from the conductive silicon substrates on which integrated circuits are fabricated. Poor electrical isolation in a receiver can lead to a local oscillator signal appearing at the output of the receiver and being effectively transmitted at the antenna. Wireless regulatory authorities limit the amount of spurious signals that can be radiated by the receiver, and limiting the amount of local oscillator radiation is important in meeting these limits.
SiO2 trench isolation and conductive guard rings are isolation techniques that have been employed (such as with SOI processes) to isolate devices of an integrated circuit. Dielectric trench isolation structures provide lateral barriers between circuit devices. Conductive guard rings are used to enclose the area to be isolated. Both techniques isolate signals and minimize the undesired coupling that would otherwise limit performance for closely spaced adjacent circuit devices.
U.S. Pat. No. 6,355,537 discloses a double ring approach where two isolation trenches (usually rectilinear) are formed around a device to be isolated. The silicon between the two isolation trenches is doped to form a conductive guard ring region, and a grounded contact is applied to the conductive guard ring region. The isolation trenches may be filled with a dielectric such as silicon oxide or oxide/polysilicon. This grounded guard ring region, the use of SOI on which the integrated circuit is formed, and the use of a high resistivity material for the substrate of the SOI greatly improves RF isolation.
However, the process disclosed in the '537 patent is primarily for thick SOI where the epitaxial layer is on the order of one micron. Moreover, the process disclosed in the '537 patent relies on an n+ buried layer that forms a low resistivity RF signal path. Also, the process disclosed in the '537 patent is BiCMOS (bipolar CMOS) dependent.
U.S. Pat. No. 5,661,329 discloses the use of a separation groove around active regions of an integrated circuit. One disadvantage of this separation groove is that external RF power can still pass through the separation groove to the active region. Moreover, this separation groove appears to be intended primarily for yield improvement and not for RF isolation. Therefore, the '329 patent does not appear to address the problem of RF isolation and appears to show no intent to terminate electric fields created by RF power.